Field of the Invention
The present invention relates to a power supply apparatus and an image forming apparatus, and particularly, to a DCDC converter apparatus.
Description of the Related Art
In a power supply apparatus, much of power loss during light-load operation is switching loss. To reduce the switching loss, an idle period of a switch element is increased to reduce the number of times of switching per unit time. However, the switching frequency decreases if the idle period of the switch element is increased. Therefore, the switching frequency enters an audible zone, and there is a problem of a groaning sound of a transformer. In a method of reducing the sound generated during standby of the power supply apparatus, a core material with a large cross-sectional area is used for the transformer to reduce a change ΔB in the magnetic flux density of the transformer. There is also a configuration of reducing the ON time of the switch element to reduce the current of the transformer per time.
An example of a unit for reducing the groaning sound of the transformer by a drive waveform of the transformer include the following configuration disclosed in Japanese Patent Nos. 3567355 and 3665984. A switching power supply apparatus that supplies electric power to a load circuit and a standby circuit includes a constant current circuit and a capacitor, and the constant current circuit raises the voltage of the capacitor at the start of the activation. The apparatus includes a soft start circuit that gradually increases the duty ratio of the switch element from zero based on the voltage of the capacitor. The soft start circuit forms a wave that gradually increases and decreases the drive current of the transformer, and the generation of sound is suppressed by reducing the change in the magnetic flux of the transformer. When the on-width of the switch element is actually set by a drive waveform that enters an idle period after gradual increase and gradual decrease, the groaning sound from the transformer is reduced.
However, when the effective cross-sectional area of the transformer is increased, or the turn ratio of the transformer is changed to suppress the change ΔB in the magnetic field density, the size of the transformer relative to the output power increases, leading to an increase in the cost. The ON time of the switching operation can be shortened to reduce the change in the current, i.e. change in the magnetic field, to reduce the groaning sound of the transformer. However, this increases the number of times of switching per unit time, and the switching loss increases. Furthermore, during the light-load operation with small output power, switching cannot be performed for a large number of times in the soft start and soft stop. To carry out the methods of Japanese Patent Nos. 3567355 and 3665984, the energy supplied in each switching operation needs to be reduced to increase the number of times of switching. Therefore, the energy per wave provided to the transformer becomes large if the switching loss is reduced by decreasing the number of times of switching, and the conventional power supply apparatus has a problem that a large sound is generated.